Reversible Assembly of Graphitic Carbon Nitride 3D Network for Highly Selective Dyes Absorption and Regeneration Yuye Zhang, Zhixin Zhou, Yanfei Shen, Qing Zhou, Jianhai Wang, Anran Liu, Songqin Liu, and Yuanjian Zhang* Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, School of Chemistry and Chemical Engineering, Medical School, Southeast University, Nanjing 211189, China S Supporting Information *
ABSTRACT: Responsive assembly of 2D materials is of great interest for a range of applications. In this work, interfacial functionalized carbon nitride (CN) nanofibers were synthesized by hydrolyzing bulk CN in sodium hydroxide solution. The reversible assemble and disassemble behavior of the asprepared CN nanofibers was investigated by using CO2 as a trigger to form a hydrogel network at first. Compared to the most widespread absorbent materials such as active carbon, graphene and previously reported supramolecular gel, the proposed CN hydrogel not only exhibited a competitive absorbing capacity (maximum absorbing capacity of methylene blue up to 402 mg/g) but also overcame the typical deficiencies such as poor selectivity and high energy-consuming regeneration. This work would provide a strategy to construct a 3D CN network and open an avenue for developing smart assembly for potential applications ranging from environment to selective extraction. KEYWORDS: graphitic carbon nitride, surface chemistry, self-assembly, reversible dye adsorption, carbon dioxide responsive
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CN may have plenty of interesting assembly behaviors and functions when considering that CN is rich in nitrogencontaining groups such as primary/secondary/tertiary amines and would work like peptides or proteins in biological systems if these amine groups could be activated. However, the possibility of utilizing CN as building blocks for responsive assembly was rarely explored, mostly because of poor interfacial properties such as very limited active groups at edges, large lateral sizes, and insolubility in most conventional solvents.15,16 Indeed, the assembly of other 2D materials such as graphene encounters similar interfacial problems. However, the issue for graphene could be addressed by tuning its lateral size and surface modification as well.17−20 As a result, lots of unique assemblies such as ultralight and superelastic graphene-based cellular monoliths were achieved.19 In this sense, pioneering works toward CN assembly have been devoted to exfoliation/ tailoring of bulk CN into nanosheets9,21,22 or quantum dots23−26 and to solubilization via intercalation,15 but few of
arbon and nitrogen are the most significant elements in the linkages of fundamental macromolecules such as base pairs for nucleic acids and peptide bond for proteins in living systems. Because of the critical role in realizing delicate biofunctions, the interfacial interactions among these macromolecules have been extensively studied, and the involved mechanisms have been applied to develop controllable assembly, nanostructures, and networks with exciting properties.1−3 Particular interests have been focused on the smart systems for a range of applications from environments to nanomedicine, for instance, capsules consisting of polyelectrolytes and supramolecular gels based on colloidal soft materials,4,5 using a variety of triggers such as light, voltage, ions, pH, and oxidants or reductants. Nevertheless, the synthesis for these smart systems usually contains multiple steps and is hard to scale-up.6 Covalently bonded carbon nitride (CN) is abundantly available by condensation of C/N-containing species such as dicyandiamide into a thermodynamic preferred 2D structure and has attracted much attention as an analogue of graphene with broad applications ranging from (photo)catalysis for sustainable energy7−12 to biosensors.13,14 It was reasoned that © 2016 American Chemical Society
Received: August 15, 2016 Accepted: September 8, 2016 Published: September 8, 2016 9036
DOI: 10.1021/acsnano.6b05488 ACS Nano 2016, 10, 9036−9043
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Figure 1. Scheme of hydrolysis process and proposed structure evolution of bulk CN treated by NaOH aqueous solution (a). NB: for clarity, only fully condensed layers of tri-s-triazine units are used, while the incomplete condensed units are omitted here. TEM image of pristine bulk CN (b) and hydrolyzed CN nanofiber (h-CN-3M, c). XRD pattern (d) and XPS N 1s (e) spectra of bulk CN and various h-CN by different concentration of NaOH solution.
changes, hydrolyzed CNs (h-CNs) under different concentrations of NaOH solution were also synthesized and denoted as h-CN-c, where c is the concentration of NaOH (0.5, 3, and 8 M). It was envisaged that the OH− would work like a scissor tailoring the 2D framework of CN into smaller units and simultaneously modify edges with functional groups such as −NHx and −OH (Figure 1a).23 To verify this speculation, the hydrolysis process was first studied by transmission electronic microscope (TEM). Figure 1b showed that the pristine bulk CN had aggregated particles with sizes around micrometers and some stacked lamellar textures at the edges. In contrast, after hydrolysis in 3M NaOH, it turned into nanofibers with a width of ca. 10 nm and a length of micrometers, and the nearly transparent feature of the nanofibers indicated its ultrathin thickness (Figure 1c). Because optical properties of nanoparticles largely depend on sizes, the h-CNs were further evaluated by UV−vis absorption and photoluminescence (PL) spectra (Figure S1). It was observed that the characteristic semiconductor absorption edge of h-CN-3M was retained but blue-shifted from 462 to 413 nm, compared with that of the pristine bulk CN. Meanwhile, PL spectra showed that the maximum emission peak of h-CN-3M also blue-shifted after hydrolysis. These changes in optical properties could be ascribed to the decreased sizes of CN basal plane and weakened delocalization of π-conjugated system after hydrol-
these nanostructured CN are further demonstrated to be successfully applied to a smart assembly system. Herein, we report that bulk CN can be simply hydrolyzed into uniform nanofibers with plenty of active groups on surfaces under alkaline condition. Interestingly, the as-prepared CN nanofibers can be reversibly assembled and disassembled into a 3D hydrogel network by using CO2 as a trigger, which has merits of high-absorbing capacity, inexpensive, nontoxic, and nonaccumulating, and it does not require the material to be transparent, compared with other stimulus such as light.27 More importantly, further taking advantage of the sophisticated intermolecular interactions in CN nanofibers and the conjugated structure of CN framework, the CN hydrogel network exhibited a high selectivity toward uptaking specific molecules and a reversible release by easy regeneration of the CN hydrogel, which well overcame the typical deficiencies such as poor selectivity and high energy-consuming regeneration of active carbon and graphene, the most widespread absorbing materials. This work would open an opportunity for bulk CN to be developed as a smart assembly of high performances for potential applications in environment and selective extraction.
RESULTS AND DISCUSSION CN nanofibers were prepared by hydrolyzing bulk CN powder in 3M NaOH solution. In order to investigate the structure 9037
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Figure 2. Sol−gel transition of h-CN (a). Proposed molecular interaction between CO2 and h-CN nanofibers (b), SEM image (c), and TEM image (d) of CN nanofiber network after freeze-drying. Steady (e) and dynamic (f) rheological behaviors of the CN hydrogel triggered by CO2.
ysis.28 Moreover, the X-ray diffraction (XRD) pattern (Figure 1d) showed that the intensity of (002) peak significantly decreased after hydrolysis, indicating the hydrolysis also made layered CN thinner and a reduced interplanar order,9,22 coincident well with the observation in TEM image and PL shift. Combining these results, it could be inferred that bulk CN was successfully hydrolyzed, and unique CN nanofiber was obtained which was rarely reported.29 To gain insight into the chemical structures of h-CN, the Fourier transform infrared spectroscopy (FT-IR, Figure S2) and X-ray photoelectron spectroscopy (XPS) (Figure 1e) were studied. The characteristic FT-IR spectrum of the h-CN-c (c =
0.5, 3, and 8 M) was mostly reminiscent of that of the bulk CN, both having a strong band at around 800 cm−1 (tri-s-triazine ring sextant out of plane bending vibration) and stretching vibration modes at 1200−1650 cm−1 (CN heterocycles), suggesting the primary framework of tri-s-triazine structure was well retained after hydrolysis. Nevertheless, compared with that of pristine bulk CN, when gradually improving the concentration of NaOH, the FT-IR spectra of the h-CN-c exhibited successive stronger peaks at 3000−3500 cm−1, which was attributed to −NHx and −OH stretching vibration modes, confirming the appearance of this functional group in hydrolysis. The high-resolution N 1s XPS spectra are shown 9038
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Figure 3. Photographs showing MB uptake and release by CN hydrogel using CO2 and N2 as triggers and cycling removal efficiency. The removal efficiency was calculated by the MB concentration difference between the solution in (2) and (6). It was worth noting that the supernatant was not readily removed completely using a pipet, thus the precipitation in (10) seemed a bit blue, but the small residue of MB almost had no side effects from the recycling. The lighter color of (9) compared with (2) was ascribed to the π−π interaction between MB and trace amounts of h-CN.
in Figure 1e, which revealed the existence of 398.1 (N1, sp2 N atoms involved in tri-s-trazine rings (C−NC)), 399.4 (N2, bridging N atoms in N−(C)3), and 400.9 (N3, N bonded with H atoms (C−N−H)). After hydrolysis, the atomic ratio of N3 to total N successively increased from 3.0% to 9.0% when gradually improving the concentration of NaOH. A similar tendency was also observed in O 1s XPS spectra (Figure S3). Consistent with the FT-IR results, it essentially suggested that layered bulk CN was successfully disintegrated with functional groups such as −NH x and −OH at surfaces under alkalinecondition, while the primary tri-s-triazine-based framework was retained. As a result of the smaller size and abundant amino and hydroxyl groups, the h-CN-c could be well dispersed in aqueous solution (Figure S4) and remained stable for at least one month. Moreover, the h-CN had a much improved BET surface area, e.g., h-CN-3M was ca. 20 times higher than the pristine bulk CN (Figure S5) and was of a hierarchical structure (Figure S6). Such improvements in interfacial properties were vital for CN to be used as a basic component for smart functional hydrogel. As is known, −NHx groups are often used for CO2 immobilization and storage, due to the effective complexation between them and carbamate formation.30,31 In this sense, in the first set of experiments, CO2 was bubbled into h-CN sol. Strikingly, it was observed that the viscosity of h-CN sol gradually increased with absorbing CO2, and finally the h-CN sol turned into a hydrogel. Such a transition was reversible by bubbling with N2 or heating the hydrogel to 80 °C to drive away CO2 (Figure 2a). It is known that the CO2 binding of the ε-amino side chain of a lysine residue at the active center of ribulose-1,5-bisphosphate-carboxylase is one of the important processes in photosynthesis.32 In this process the CO2 is bound covalently, resulting in the formation of a carbamate
functionality (N−C(O)−O). Moreover, quite a few examples of compounds show that amine functionalities are able to chemically bind CO2.4 In this regard, the transition between h-CN-3M sol and hydrogel evidently suggesting a delicate complexation between CO2 and functional − NHx groups on h-CN-3M occurred and effectively cross-linked hCN-3M into a network (see scanning electron microscope (SEM) image in Figure 2c, TEM image in Figure 2d ,and the proposed complexation mechanism in Figure 2b). In order to figure out the chemical reaction between CO2 and h-CN-3M, FT-IR spectra were measured on a FT-IR spectrometer with a homemade attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) rack.33 To eliminate the interferential signal from environmental molecules, the background spectra which contain a whole signal of h-CN-3M, water, and background from instrument were collected and subtracted from the sample spectra. From Figure S7, no absorption signal was detected, as shown in the spectra before bubbling CO2. However, a weak absorption peak was observed at 1635 cm−1, which was assigned to the carbamate asymmetric stretch vibration.31,34 Thus, CO2 tends to bind covalently with h-CN-3M, resulting in the formation of a carbamate functionality. The mechanical properties of CN hydrogel were studied by rheological measurement. Steady mechanical rheometry (Figure 2e) showed the shear-thinning characteristic of CN hydrogel triggered by CO2. The hydrogel viscosity decreased with the increase of shearing rate, a typically shear-thinning phenomenon, indicating the sufficient crosslinks between CN and CO2 molecules to maintain the network in CN hydrogel.35 Small deformation oscillatory measurements (Figure 2f) demonstrated that storage modulus (G′) was higher than loss modulus (G″) over the entire angular frequency range (1−100 rad/s), implying the substantial elastic response and 9039
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Figure 4. Photographs and relative concentration (a) of the supernatants of centrifuged hydrogel mixed with different dye solutions. From left to right, 0.5 mL of MB, azure B, acriflavine, safranin O, RhB, eosin Y, and MO (100 mg L−1) was added to 0.5 mL of h-CN-3M (3 mg/mL), respectively. The phenomenon that the concentrations of eosin Y and MO were slightly higher than initial concentrations was ascribed to the negative charge repulsion between CN nanofiber and dyes. Photographs of dyes (from left to right, MB/RhB, MB/eosin Y, MB/MO) mixed with h-CN-3M hydrogel and selective adsorption of dyes by CN hydrogel (b). UV−vis absorption spectra of MB, RhB, and that with additional CO2 and h-CN-3M in aqueous solution (c).
permanent network of CN hydrogel.3 The complexation of CO2 and h-CN indicated the possibilities of CO2 capture or detection by h-CN and also the fabrication of 3D CN from the CN hydrogel triggered by CO2, which was hardly prepared according to the previous reports.36,37 Nevertheless, it should be noted that the CN hydrogel was stable in air, but after the solvent exchange and drying, it was found that the as-obtained CN aerogel was easily collapsed, indicating that the interfacial interaction between adjacent CN nanofibers linked by CO2 in hydrogel was dynamic and delicate. Unlike the 2D graphene oxide that has a large contact area between neighboring nanosheets and that can from a robust 3D graphene-based network after drying, the joint of the two CN nanofibers was much smaller. In this sense, if we would like to have a robust CN network after drying, a stronger connection between two CN nanofibers is needed. CO2 and other compounds consisting of oxygen-containing anions such as NaHCO3, poly(acrylic acid) , adipic acid, and acetic acid could also turn the h-CN sol into a hydrogel network (Figure S8), presumably by a similar complexation interaction like CO2. Nevertheless, among them, CO2 was most easily removed from the CN hydrogel by bubbling N2, making the transition between sol and hydrogel reversible without increasing the chemicals in the system, an extra benefit for a smart responsive system. Since solids are more readily quantified than gas and the saturation concentration of CO2 in solution is low, NaHCO3 was used instead of CO2 to evaluate the influence of concentration on the gelation of CN sol. It was found the effective gelation (3 mg/mL of h-CN-3M, pH = 6.5) occurred at a certain concentration of NaHCO3 (1.5−8 mg/mL, pH = 7.0−8.0). It suggested that such a delicate complexation interaction was in a narrow pH region, which would greatly benefit a highly sensitive switching between sol and hydrogel for a smart responsive system. More interestingly, the as-prepared CN hydrogel network exhibited a favorable adsorption behavior toward methylene blue (MB). When 1 mL of MB aqueous solution (0.1 mg/mL) was added on the top of 1 mL of CN hydrogel (8.9 mg/mL) at 25 °C, most MB molecules were adsorbed by the CN hydrogel (Figure S9a) after 24 h, and just 3.5% of MB was detected in the original solution by absorption spectra (Figure S9b). The maximum adsorption capacity was estimated to be at least 402 mg/g (see Figure S10 and experiential details in SI). This uptake capacity was competitive with the best previously reported supramolecular gel nanomaterials (the reported
typical values
